Host aggregate configuration throws exception

If the virtualization driver did not implement the host
aggregate support functions (add_to_aggregate,
remove_from_aggregate and undo_aggregate_operation) then an
exception is thrown when a host aggregate was created/deleted.
Configuration of the host aggregates breaks tempest tests when
the underlying driver does not have these methods implemented.

The only driver that actually uses this is the Xen driver.
The solution enables all drivers to support this without
having to add in specific code. More specifically, the compute
manager will not raise NotImplementedError.

Closes-Bug: 1229912
Closes-Bug: 1229911

Change-Id: Iee69e45d94047742b6a499a139bac96ad9dd971f
Merge "Host aggregate configuration throws exception"